The silicon transistor technology on which modern computation is based has shown rapid exponential improvement in speed and integration for more than four decades. It is widely expected that this rate of improvement will slow as device dimensions approach the nanometer scale (see J. D. Meindl et al., “Limits on silicon nanoelectronics for terascale integration”, Science 293, pp. 2044-2049, 2001). This has led to the exploration of many alternative computation schemes, most of which are based on gating the flow of electrons by novel means such as quantum dots (see D. Goldhaber-Gordon et al., “Overview of nanoelectronic devices”, Proc. IEEE 85, pp. 521-540, 1997), organic molecules (see G. Y. Tseng et al., “Toward nanocomputers”, Science 294, pp. 1293-1294, 2001), carbon nanotubes (see S. J. Wind et al., “Vertical scaling of carbon nanotube field-effect transistors using top gate electrodes”, Appl. Phys. Lett. 80, pp. 3817-3819, 2002), and the motion of single atoms or molecules (see Y. Huang et al., “Logic gates and computation from assembled nanowire building blocks”, Science 294, pp. 1313-1317, 2001). Other proposals include electrons confined in quantum dot cellular automata (see C. S. Lent et al., “A device architecture for computing with quantum dots”, Proc. IEEE 85, pp. 541-557, 1997; and I. Amlani et al., “Digital logic gate using quantum-dot cellular automata”, Science 284, pp. 289-291, 1999), magnetic dot cellular automata (see R. P. Cowburn et al., “Room temperature magnetic quantum cellular automata”, Science 287, pp. 1466-1468, 2000), and solutions of interacting DNA molecules (see R. S. Braich et al., “Solution of a 20-variable 3-SAT problem on a DNA computer”, Science 296, pp. 499-502, 2002).
Computation can also be achieved using purely mechanical means (see D. D. Swade et al., “Redeeming Charles Babbage's mechanical computer”, Scientific American, pp. 86-91, February 1993; and K. E. Drexler et al., Nanosystems, John Wiley & Sons, New York, 1992). It has been widely assumed that mechanical devices will always be too large to be competitive with electronic computational devices. Atomic scale mechanical devices capable of performing logic computations would, however, be of interest to the computational technology community.